Low-light Lightning Detections in the Jovian Atmosphere by Juno’s Advanced Stellar Compass

Low intensity phenomena on Jupiter in the visual range, e.g. aurora and lightning, are readily visible on the nightside of the gas giant. The highly elliptic orbit of NASA’ Jupiter probe, Juno, provide perijove distances in the range of 5-10,000km, offering unique close range observation opportunities, when the probes optical instruments happens to be pointed towards dark regions of the planet.

The micro Advanced Stellar Compass (µASC), an instrument onboard Juno primary purpose is to serve as an attitude reference for the Juno Magnetic Field investigation, provides accurate bias free attitude information continuously throughout the prime mission. The µASC uses a set of four optical sensors that are optimized for low-light imaging, which enables detection of stars and objects as faint as 7-8Mv.

The 13° angular offset  between the star tracker cameras’ axis  and Juno’s spin axis (in anti-sun direction), routinely places the Jovian night-side high latitude regions into the field of regard of the star trackers. This peculiar geometry facilitates imaging of the low light phenomena, such as lightning and aurora, at large slant angle offering unique altitude information of the upper atmosphere phenomena imaging as well as its localization. The star tracker images further offer star occultation observation enabling profiling the density profile of the upper atmosphere.

We revisit captured ASC image data, extracting  observations of lightning events and attitude determinations, to estimate the lightning altitude in the Jovian atmosphere. The altitude at which lightning occurs in Jupiter’s atmosphere is key to an ammonia-water dynamical system that is thought to explain the puzzling depletion of ammonia observed by the Microwave Radiometer investigation.